In spacecraft control systems, the ability to provide accurate and robust attitude determination and control enable spacecraft in orbit to counter disturbances in nominal operation and to achieve mission-specific requirements. The accuracy and precision requirements are particularly challenging for small satellites (and other spacecraft) where limited volume, mass, and power are available. Attitude knowledge of small spacecraft is often achieved using devices such as sun sensors and magnetometers. However, these sensors have various limitations. Sun sensors, for example, lose their functionalities in periods of eclipse in orbit. Magnetometers, as another example, cannot acquire high accuracy attitude measurements due to the constantly changing Earth magnetic field. Earth horizon sensors (EHSs) have emerged as an efficient and relatively inexpensive means for providing relatively precise attitude determination, capable of satisfying attitude knowledge requirements of small spacecraft in low-Earth orbit (LEO), especially for missions with Earth-specific science objectives.
While the Sun and stars are effectively point sources from the perspective of a spacecraft in LEO, Earth appears as a large and bright target that is easily detected. For a spacecraft in LEO, Earth subtends a solid angle significantly wider than the solid angle of the Sun and of Betelgeuse (the ninth-brightest star in the night sky). Due to the large expanse of Earth in the spacecraft-centered unit sphere, detection of the horizon is required for precise attitude knowledge. Horizon sensors provide the primary means to directly determine the spacecraft's attitude with respect to Earth.
Infrared Earth horizon sensors detect Earth's electromagnetic radiation in the infrared spectrum, caused by the Sun's radiation being absorbed and re-radiated by Earth's surface and atmosphere. In the long-wave infrared spectrum beyond 4 μm, Earth becomes a dominant infrared radiation source, exceeding the Sun irradiation level by several orders of magnitude. Infrared radiation is often referred to as thermal radiation due to the thermal energy generated by the emission of electromagnetic radiation in this spectrum. The thermal energy emitted by Earth can be measured using thermopile detectors, devices that convert thermal energy collected in the sensor's field of view (FOV) into electrical energy. Commercial thermopile sensor units generally have Gaussian sensitivity, with the half-width at half-maximum (HWHM) defined as the effective half-angle FOV.
While large spacecraft often have EHS on scanning wheels, it is more practical for small spacecraft to have fix, body-mounted EHS system due to mass, volume, and power limitations. Thermopiles can be mounted at various locations with fixed and predetermined directions, depending on the mission altitude and sensors' FOV. Arrays of thermopiles have been utilized to maintain nadir pointing by ensuring zero temperature difference between sensors in each sensor pair along the velocity vector and side directions. To fully determine the spacecraft's attitude in an inertial frame through the TRIAD method, a full expression of the nadir vector in the spacecraft's body frame is needed. The second reference vector used in the TRIAD method can be the Sun vector, acquired by sun sensors during daytime, or the magnetic field direction, which can be determined using magnetometers during periods of eclipse.